This application is based on an application No. 10-299391 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
This invention relates to an electrodeless discharge lamp which prevents occurrence of devitrification and to an electrodeless discharge lamp apparatus which uses the electrodeless discharge lamp.
(2) Description of the Prior Art
One example of conventional electrodeless discharge lamps is disclosed in Japanese Laid-Open Patent Application No. 9-120800 in which indium halide is used as the luminescent substance. Hereinafter, electrodeless discharge lamps in which indium halide is used as the luminescent substance are referred to as indium lamps. The indium lamps have excellent color rendering properties and highly efficient optical properties since continuous spectrum of light is radiated by molecules of indium halide. Especially, it is known that indium bromide (InBr) used as the indium halide provides high luminous efficiency (xe2x80x9cNovel High Color Rendering Electrodeless HID Lamp Containing InX,xe2x80x9d A. Hochi, M. Takeda, S. Horii, T. Matsuoka, IDW ""96 proceedings, PP 435-438).
Typically, metal halide lamps having electrodes (hereinafter referred to as metal halide lamps) are each composed of a plurality of different luminescent substances to improve the luminous efficiency and the color rendering properties. As a result, as such a metal halide lamp continues to emit light and the luminescent substances are consumed, the ratio between the luminescent substances existing in the arc tube changes and the luminescent color also changes. On the contrary, the indium lamp, an electrodeless discharge lamp, which is filled with only one luminescent substance, can achieve the same or higher luminous efficiency and color rendering properties as metal halide lamps, and are hard to cause change in the luminescent color thereof.
However, conventional indium lamps have a problem that that compared to electrodeless discharge lamps in which sulfur is used as the luminescent substance (hereinafter referred to as sulfur lamps) the devitrification occurs quickly to quartz glass, the material of the arc tube, and decreases the light emission and reduces the lamp life. The sulfur lamp is disclosed in, for example, Japanese Laid-Open Patent Application No. 6-132018. For example, a continuous lighting test conducted for conventional indium lamps showed that after the test pieces are lighted for 10,000 hours, the devitrification develops in about one thirds of the area inside the arc tube in extreme ones among the test pieces.
It is therefore an object of the present invention to provide an electrodeless discharge lamp which prevents the occurrence of the devitrification and has a long life and an electrodeless discharge lamp apparatus which uses electrodeless discharge lamp. The following are details about how the inventors reached the present invention.
The devitrification of conventional indium lamps is unique compared to that of conventional metal halide lamps: the devitrification of conventional indium lamps turns the color of the arc tube to yellow, while the devitrification of conventional metal halide lamps turns the color to white. Also, the region where the devitrification occurs in conventional indium lamps belongs to neither the maximum temperature region nor minimum temperature region of the arc tube. Furthermore, no substance that is highly reactive with quartz glass, the material of the arc tube, can be contained in the arc tube. From these points, it is presumed that the devitrification of conventional indium lamps occurs through different processes from that of conventional metal halide lamps.
The inventors of the present invention analyzed conventional indium lamps and reached unique findings concerning the devitrification of the conventional indium lamps by reviewing the results of the analysis, and completed the present invention based on the findings. Now, the analysis results will be described together with the processes through which the inventors assume the devitrification occurs to indium lamps.
First, conventional indium lamps were lighted so as not to develop the devitrification, then precipitation of indium (In) was observed inside the wall of the arc tube during and after the lighting. The region on the arc tube where the precipitated indium appeared was where the wall temperature was relatively high, not where the wall temperature was the lowest. Grains of indium with the diameter of up to about 20 xcexcm were observed to be precipitated on the surface of the wall. Also, crater-like indentations, which are assumed to be formed as the quartz glass melts, were observed in the region where the precipitation appeared.
Next, concerning the conventional indium lamps to which the devitrification had occurred, the region of the arc tube where the devitrification occurred was observed under magnification. It was found through the observation that a lot of crater-like indentations whose size is equivalent to the indium grains attached to the same region were formed in the region, and that quartz glass had partially been crystallized in the region. The crystallized portion of the quartz glass was analyzed in the direction of depth to find that indium and bromine (Br) were equally distributed in the crystallized portion. This is considered to be the reason why the devitrification part is yellow.
It summarized from the above that the crater-like indentations are formed by the heat of the precipitated indium, and that crystallization of quartz glass is also promoted by the heat.
It is concluded from the above analysis and consideration that the devitrification occurs to conventional indium lamps as follows.
In conventional indium lamps, argon (Ar), a gas for starting-up, contained in the lamps starts discharging to generate high-temperature arcs, the temperature of the wall of the arc tube easily rises, indium bromide being a luminescent substance quickly evaporates, and atoms of indium and bromine dissociated from the indium bromide or molecules of indium bromide in the high-temperature arcs are excited and perform luminous discharge. The high-temperature arcs exist even near the wall of the arc tube. That means, there are atoms or ions having high energy near the wall of the arc tube. As a result, there is a high possibility that the dissociated atoms of indium and bromine contact the arc tube wall before they recombine. When this happens, what is called halogen cycle in which the metal halide dissociates at high temperatures and recombine at low temperatures in cycles hardly happens, and this tends to create a state in which indium and bromine are liberated from each other. The inventors actually confirmed through a test lighting of less than one hour that the contents of the arc tube after the lighting partially contained indium and bromine liberated from each other.
Indium as a simple substance has a high boiling point of 2080xc2x0 C. As a result, when atoms of indium reach the arc tube wall, they tend to attach to the wall even if the wall has a relatively high temperature. The indium attached to the arc tube wall during the lighting of the indium lamp is heated to a high temperature by the energy applied to the lamp for the lighting. When indium as a simple substance is heated, it passes a high temperature which can easily melt quartz glass before indium reaches its high boiling point. With such a high temperature, quartz glass softens or melts and tends to generate a crystalline nucleus. The generation of the crystalline nucleus facilitates development of the crystallization of quartz glass around the crystalline nucleus. The crystallization of quartz glass develops to the devitrification.
On the contrary, it is considered that such devitrification does not occur to sulfur lamps. This is because the luminescent substance, sulfur, has a low boiling point and a high vapor pressure.
Up to this point, the processes through which the inventors assume the devitrification occurs to indium lamps have been described. The inventors have found from these facts that the occurrence of the devitrification to conventional indium lamps can be restricted by preventing the attachment of atoms of indium to the arc tube wall. This led the inventors to the achievement of the electrodeless discharge lamp of the present invention which prevents occurrence of the devitrification. It is needless to say that the gist of the present invention can also be applied to lamps in which a material other than indium halide is used as the luminescent substance.
The above object is achieved by an electrodeless discharge lamp including: an arc tube in which a luminescent substance and a melting-interruption substance are enclosed, where the luminescent substance is a metal halide, and the melting-interruption substance interrupts elements of a metal constituting the metal halide in melting a wall of the arc tube, the elements being dissociated from the metal halide.
With the above construction, the wall of the arc tube is prevented from being melted by a metal dissociated from a metal halide used as a luminescent substance. This prevents the occurrence of the devitrification, and enables an electrodeless discharge lamp apparatus using this electrodeless discharge lamp to have a long life.
In the above electrodeless discharge lamp, the melting-interruption substance may be a metal halide, and a ratio of: a summation of the number of moles in a halogen included in the melting-interruption substance and the number of moles in a halogen included in the luminescent substance; to a summation of the number of moles in a metal included in the melting-interruption substance and the number of moles in the metal included in the luminescent substance is in a range of more than 1.02 and less than 1.07.
In the above construction, it is preferable that the ratio is in a range of 1.03 and 1.05.
In the above electrodeless discharge lamp, the melting-interruption substance may include: the metal included in the luminescent substance; and the halogen included in the luminescent substance, and composition of the melting-interruption substance is different from composition of the luminescent substance.
With the above construction, a possibility of the change in the lamp characteristics such as the luminescent color is reduced. Also, compared to a case in which a halogen as a simple substance is added, the above construction facilitates the control of the ratio between the metal and the halogen in terms of the number of moles in total.
In the above electrodeless discharge lamp, the luminescent substance may be a metal halide including a halogen and a metal which is an element belonging to the Periodic Table group IIIB, and the melting-interruption substance is a simple substance of the metal included in the luminescent substance.
The above object is also achieved by an electrodeless discharge lamp apparatus including: an electrodeless discharge lamp including an arc tube in which a luminescent substance and a melting-interruption substance are enclosed, where the luminescent substance is a univalent metal halide which includes a halogen and a metal being an element belonging to the group IIIB, and the melting-interruption substance interrupts elements of the metal constituting the metal halide in melting a wall of the arc tube, the elements being dissociated from the metal halide; and an excitation energy supplying means for supplying to the electrodeless discharge lamp an energy for exciting the luminescent substance to emit light.